In the past a number of additives to the MgO powder were introduced which were to improve the formation of the insulating layer and the magnetic properties of the finished product. These include titanium oxide (TiO.sub.2), boron compounds such as boron oxide (B.sub.2 O.sub.3) or sodium tetraborate (Na.sub.2 B.sub.4 O.sub.7) as well as antimony compounds such as antimony sulphate (Sb.sub.2 (SO.sub.4).sub.3) in combination with a chloride, preferably antimony chloride SbCl.sub.3. The additives used frequently also showed disadvantages in addition to the positive influences on the respective target values, which reduced the product quality. In summary, the processing of such additives is complicated because they partly have to be dissolved in previously heated water. Particularly in the salts of sodium tetraborate and in particular antimony sulphate, which are difficult to dissolve in water, undissolved coarse particles will lead to inhomogeneities in the antisticking layer and thereafter to local defective places in the glass film. With respect to antimony sulphate it has to be taken into account in addition that the compound is expensive and graded within the category of "low-poisonous" substance. An inhomogenous distribution of titanium oxide in the antisticking layer will lead to defective places in the glass film.